This invention relates to an electrophotographic process and an apparatus for printing or duplicating one or more copies by carrying out the electrophotographic process.
The prior art electrophotographic process may be classified into the following two types: one for obtaining one duplicated copy with a single imagewise exposure (hereinafter referred as single imagewise exposure-single copy type) and the other for obtaining a plurality of duplicated copies on the basis of a single and same image information once formed by a single imagewise exposure (hereinafter referred as single imagewise exposure-multiple copy type). The latter type of electrophotographic process is described, for example, in U.S. Pat. Nos. 2,951,443, 3,598,580 and 3,627,523. According to these processes, the development and transfer steps are successively repeated for an electrostatic charge image once formed on a charge retentive member without destroying the charge image so as to form the same image on a plurality of image receiving papers. Another process which belong to said latter type of electrophotographic method is described in U.S. Pat. Nos. 2,756,676 and 3,713,734. According to the process described in said U.S. Pat. No. 2,756,676, in order to obtain a plurality of duplicated copies a toner image developed on the charge retentive member is partially left upon transferring the toner image onto the record medium and then steps of uniform charging, uniform exposure, development and partial transfer are repeated successively. That is, a plurality of copies are formed on the basis of the residual toner image after transfer. According to the process described in said U.S. Pat. No. 3,713,734 use is made of a photoconductive screen and a flow of corona ions are repeatedly modulated on the basis of an electrostatic latent image formed on the photoconductive screen so as to obtain a plurality of duplicated copies.
Further, in the electrophotographic art various techniques for changing a development density and a half tone reproduction property of a duplicated copy image have been developed and some of them are practically used. According to the most commonly used technique for changing the development density of the copy image an input signal energy such as an amount of image projection light for forming an electrostatic latent image corresponding to an original image on the charge retentive member is controlled. This method can be effectively applied to said electrophotographic apparatus of single imagewise exposure-single copy type, but can not be applied to said electrophotographic process of single imagewise exposure-multiple copy type. That is, in case of forming a plurality of duplicated copies of a single and same original by means of the former process when a first copy having an incorrect development density is obtained, said input signal energy such as an amount of imagewise exposure light can be suitably adjusted so as to obtain the subsequent copies having a desired density. On the contrary, in case of the latter process namely the electrophotographic process of single imagewise exposure-multiple copy type, when the development density of the first duplicated copy is not correct, the input signal energy for producing the electrostatic latent image could not be corrected during the process for printing the subsequent copies, because in this process the input signal energy is not applied after the first duplication. Therefore, in this case all of the copies obtained might have an incorrect density.
As the technique for adjusting the half tone reproducibility it has been proposed to decrease or increase an initial potential uniformly charged on the charge retentive member so as to decrease or increase an electrostatic contrast of the electrostatic charge latent image formed thereon. This method may be applied to an electrophotographic apparatus having a particularly high electrostatic contrast for obtaining a copy image having a low contrast such as a photographic image. However, in this method, since the uniformly charged potential is made low the highest density of the duplicated copy is also limited to a low value and further if in order to lower the uniform charging potential the corona voltage applied to a corona charging device is lowered, the corona charging becomes unstable and thus a non-uniform charging might be produced, which causes an uneveness in the developed image.
Then, the inventor has proposed in Japanese Patent Application No. 104,223/76 an electrophotographic apparatus which can change the density and the half tone reproduction property of successive duplicated images. This electrophotographic apparatus makes use of, as the the charge retentive member a photosensitive screen composed of an insulating layer, two electrically conductive layers coated on opposite surfaces of the insulating layer and a photosensitive layer coated on one of the electrically conductive layers and is so constructed that a bias voltage applied between said two electrically conductive layers can be controlled from the outside so as to adjust the density and the half tone reproduction property of the successive picture images. However this technique can be only applied effectively to the electrophotographic apparatus comprising the photosensitive screen having the two electrically layers but can not be applied to the electrophotographic apparatus comprising the charge retentive member other than the photosensitive screen or a photosensitive screen having only one electrically conductive layer.